DRY-up the database configuration file
DRY-up the database configuration file

This change moves common config keys into a 'default' block which is reused across the development, test and production database configurations.

This makes it easier to spot configuration that remains the same between different environments, and avoids having to maintain multiple copies of it.

alphagov/whitehall,alphagov/whitehall,alphagov/whitehall,alphagov/whitehall